Nitride semiconductors have a bandgap energy in the range of 0.7-6.2 eV, and can be widely used for light emitting devices, high frequency devices and high power devices.
For instance, in the case of using a material of InxGa1-xN (0≦x≦1) for a light emitting device, by changing the In composition ratio x, the bandgap can be controlled to between 3.4 eV of GaN and 0.7 eV of InN. Thus, a high brightness visible light emitting device can be obtained.
However, there is no substrate material which is easily lattice matched with and close in thermal expansion coefficient to nitride semiconductors. Thus, sapphire is often used, which has high stability in the crystal growth atmosphere and has high volume productivity.
In this case, strain due to difference in lattice constant occurs. Furthermore, strain due to difference in thermal expansion coefficient occurs in the temperature decreasing process after crystal growth. Thus, a nitride crystal layer includes many crystal defects.
On the other hand, a step portion provided in the substrate facilitates improving the characteristics of the semiconductor device. For instance, in a light emitting device, concave-convex structure provided at the surface of the substrate can change the traveling direction of light, and can increase the light extraction efficiency. However, a step portion provided at the surface of the substrate may increase the crystal defect density and cause the problem of degrading the characteristics of the light emitting device.